Deposition of materials is used for purposes such as microfabrication, substrate treatment, coating, or others. For example, Parylene (poly-para-xylylene) is a family of organic, dielectric, polymer materials that may be deposited by chemical vapor deposition (CVD) methods under room temperature, or slightly above or below room temperature. It is an exceptionally conformal coating material with very low pin-hole density. Parylene is particularly useful for sealing or insulating, and can be used for fabricating and/or treating devices such as semiconductor, telecommunication, micromachining, and micro-electromechanical (MEMS) devices. It is also useful for purposes such as document preservation.
In fabrication or treating processes involving deposition of Parylene or other materials, it is often important to monitor the deposition thickness to control the thickness. For example, a Parylene film may be employed to function as part of a mechanical structure, such as a sensor or an actuator. In this case, the thickness of the film is an important parameter in determining the performance specifications of the structure.
Typically, Parylene deposition thickness is determined by preloading a deposition system with a controlled amount of solid-phase dimer materials. The thickness is controlled either by the amount (weight, for example) of polymer dimer that is loaded into a vacuum sublimation chamber, where Parylene deposition is typically conducted, or by the time of the deposition process. Many industrial processes use the former control method. When an amount of dimer is used to predict or control the deposition thickness, the endpoint of the process is determined by the moment the preloaded dimer is exhausted; that is, at the complete sublimation of all of the dimer materials.
However, these conventional methods do not reliably predict or control the actual deposition thickness. There can be a significant run-to-run variance of deposition thickness for a certain deposition time or pre-loaded dimer amount, for example, due to changing baffle geometry from prior deposition. Variation on the order of 10-40% has been commonly observed. In addition, for large deposition chambers (often used in industry), the thickness may also vary from location to location with a significant overall variance, typically between 5%-10%. This variance may be acceptable for general coating and sealing applications, however it is too great for certain applications, such as many MEMS applications, where materials such as Parylene form integral mechanical components. Inaccuracy of predicting and/or controlling deposition thickness is especially problematic when the planned deposition thickness is relatively small, for example, less than 5 μm.